The V.90 modem is also known as the 56K modem, which, due to power limitations imposed by the FCC is presently limited to 53 Kbits/second transmission rate. FIG. 4 is a simplified block diagram illustrating how a V.90 modem 530 may be connected to a server 510 through a codec 520. V.90 modem 530 is an analog modem communicating with a telephone company (telco) codec (coder/decoder) 520 through a local dial-up line or local loop 540.
As may be appreciated by one of ordinary skill in the art, such a local loop 540 may contain analog impairments characterized as “loop loss”. V.90 modem 530 may be provided with an equalizer to offset some of this loop loss. Data transmitted from V.90 modem 530 to codec 520 may be in one of a number of formats depending upon the type of codec.
For example, a so-called μ-law codec may receive data in a 13-bit format (as illustrated in FIG. 4). A so-called A-law codec may receive data in a 12-bit format. Other, so-called “non-conforming” codecs may receive data in yet other formats. Regardless of which format the data is received in, codec 520 converts data received-from V.90 modem 530 into digital form (typically 8 bits) for transmission over a telco digital trunk line 550 to server 510.
Digital trunk line 550 may itself contain so-called “digital” impairments, including robbed bit signalling (RBS), digital pad (PAD) and inter-modulation distortion (IMD). Most of these digital impairments are-due to design considerations implemented in the telco digital network when it was largely used as a voice-only network. However, such impairments present problems in transmitting digital data over such a network.
The V.90 standard adds to and inherited advantageous features of pre-V.90 56K modems (e.g., X2 modem and Kflex modem). One important feature of the V.90 modem is performance optimization. Using Digital Impairment Learning sequence (DIL) data, the client modem is capable of generating optimal constellations to achieve best throughput for given conditions.
V.90 constellations rely closely on DIL data points. Thus, accuracy of DIL data is one key for accurate constellation generation. One big problem of the prior art is how to obtain highly accurate and optimal DIL data points. V.90 modem 530 must receive a training signal (the Digital Impairment Learning signal, or DIL) and be able to distinguish analog impairments from digital impairments (“de-noising” data) in order to properly characterize the data channel and generate accurate and optimal data constellations.